In recent years, it is demanded to increase functions of a node for constructing a photonic network, and not only a function for point-to-point transmission but also functions for freely switching a path of an optical signal such as an optical add/drop multiplexing (OADM) function and a wavelength cross-connect (WXC or optical hub) function are demanded.
Particularly in the optical add/drop multiplexing function, Colorless Mini OADM (hereinafter referred to simply as colorless OADM) is demanded. The colorless OADM has a function of making it possible to add (that is, insert) and drop (that is, branch) an optical signal of an arbitrary wavelength from each port although the number of Add and Drop ports is as small as, for example, eight and is smaller than the number of wavelengths to be multiplexed, and it is expected that the colorless OADM contributes to improvement of the network (wavelength routing), enhancement of the flexibility of apparatus arrangement (reduction in space by reduction of mounting slots) and reduction in cost.
It is to be noted that the following document discloses a known prior art relating to the invention of the present application:    [Patent Document 1] Japanese Patent Laid-Open No. 2006-87062
In the colorless OADM, in order to achieve reduction in cost, different from an ROADM (reconfigurable practical add/drop multiplexer), a function of adding and dropping wavelength multiplexed signals for all channels is not demanded, and it is necessary to suppress the number of wavelength selection switches of a high cost. On the other hand, in order to attain a transmission over a plurality of spans with favorable quality, it is also demanded to efficiently suppress the crosstalk between adjacent channels.